The present invention relates to a method for preparing the surface of a high temperature brazing alloy for additional braze.
In general, brazing may be defined as a metal joining process wherein the metals to be joined are heated to a temperature below their melting points and joined together by a non-ferrous filler metal, i.e., a braze, having a melting point of not lower than 800.degree. F.
Such techniques, especially when a high temperature brazing alloy, such as a standard gold brazing alloy is employed, provide remarkably high bonding strength between the two metals that are joined. Consequently, such techniques have found extensive use in precision bonding situations, such as in the brazing of the vanes on rotors and stators of jet engines and the like.
While the use of brazing techniques such as described above has proved highly satisfactory, occasionally after extended use of an aircraft type engine cracks do appear in the brazing alloy used to bond parts together due to vibration. In such cases, it has been customary to break the bond between the joined parts by removing all of the braze and then rebrazing the parts together again. Such action is expensive and time consuming.
In accordance with the present invention a method is provided whereby a cracked or damaged braze is treated in place so as to be receptive to further brazing wherein said cracks are filled and the bond accordingly strengthened to the point of initial acceptability.
It is an object of the present invention to provide a method for filling cracks in braze.
It is another object of the present invention to provide a method of surface preparation for brazing.
It is a further object of the present invention to provide a method of repairing damaged brazed bonds joining two metal parts without the necessity of completely removing the braze.
The above and further objects and advantages of the present invention will become apparent in view of the following discussion.
As mentioned above, the basic tenet of the present invention is to prepare a surface for brazing. In particular, it involves the preparation of a damaged or cracked braze for rebrazing, wherein the damage or cracks may be filled with additional brazing alloy.
There are four basic steps for carrying out the method of the present invention: (1) cleaning, which includes the removal of excess braze and any oxides thereof from the adjacent parts: (2) electrolessly pre-plating the damaged or cracked surface; (3) electrolytically plating the damaged or cracked surface with a metal which is a component of the braze; and (4) cleaning the treated damage and/or cracks so as to remove any residue which might interfere with the rebrazing.
The first step of the method is to clean the braze and the adjacent parts area. In doing this, excess braze and oxides thereof are first removed by mechanical and/or chemical processes, such as those that are described in U.S. Pat. Nos. 3,819,494 and/or 3,958,984 to Laurence R. Fountain. Once the excess braze and any oxides thereof have been removed, the part is further cleaned with alkaline and acidic metal surface cleaning techniques which are well known in the art and which will be further described below.
The second step of the method is basically described as a pre-plating of the walls of the crack and/or damaged area with a metal which is a component of the braze material being repaired. While the pre-plating is done with a metal which is a component of the braze being repaired, it is essential that the pre-plating metal layer be deposited by electroless plating methods to insure penetration into the damaged area of the braze. However, in view of the expense involved in electroless plating, minor strikes, i.e., electroplating, may also be employed along with the electroless plating in order to inexpensively build up the pre-plate to a desired thickness and to activate the surface of the damaged area to be repaired.
If the electroplating is employed, the total deposit thereof amounts to only about 50 millionths of an inch and the deposit of the electroless plating is from about 0.0002 - 0.0005 inch.
The third step of the process is a plating operation with a metal which is a component of the braze being repaired. Basically, a standard plating bath is employed using electroplating methods. The metal deposit is preferably from .0002 - .0005 inch, and it acts as a wetting agent or carrier for the braze used in the final repair.
The last step of the process of the present invention prior to the actual repair with the brazing alloy is a cleaning step. Primarily, said cleaning involves the removal of any salt residue which may have been formed during the preceding step.